windsprite wrote:I'm really not a tech-y person, so I don't know a lot about photosites and all that, but from a practical standpoint, I think you can only pile photons so high on a sensor. To collect more of them, you have to spread out over a larger area. I vaguely remember reading of things like well depth which affect efficiency, but it seems to me that total area must be by far the more significant factor, which is I think what Bob and the other gents have been saying.

Interestingly, it has to do with design. Market expectation is that base ISO will be 100, sometimes 200, and the nominal exposure for that ISO plus the sensor size determines how much light is collected. Four Thirds manufacturers could have designed for a base ISO of 25, in which case a FT camera could have collected as many photons as a FF one, and had the same peak image SNR\

Naturally, there's a catch, else APS-C and FF sensors would also have ISO 25.

It's not really a catch, it's how you design a camera for some maximum feasible lens aperture, which is usually determined by size and cost constraints. Strangely, when listed by angle of view, that maximum feasible is strangely constant.